Results from the Kepler [1]}, [2]} and K2 [3]} missions have revealed that M dwarfs (\(T_\textnormal {eff} \lesssim 4\,000\)  K) host on average \(\sim \) 2.5 planets with radii below 4 \(R_\oplus \)  and with periods of less than 200 d [4]}, [5]}.
The bimodal distribution of radii of small, close-in planets produces a gap that separates planets with 1–2 \(R_\oplus \) , the so-called super-Earths, and 2–4 \(R_\oplus \) , denominated as mini-Neptunes.
For solar-like stars, this radius gap occurs between 1.7 \(R_\oplus \)  and 2.0 \(R_\oplus \)  [6]}, [7]}, [8]}, and it resides a bit lower for low-mass stars, between 1.4 \(R_\oplus \)  and 1.7 \(R_\oplus \)  [9]}, [10]}.
This bimodality suggests that mini-Neptunes are mostly rocky planets that were born with primary atmospheres and were able to retain them, whereas planets below the radius gap lost their atmospheres and were stripped to their cores [11]}.
